Temperate cereals such as wheat and barley deposit temporary carbon reserves in the stem and leaf sheath at the early reproductive stage in the form of water soluble carbohydrates (WSC). WSC in wheat stem is composed of fructans, sucrose and hexoses with fructans being the major component at anthesis (Xue et al., 2008b; Ruuska et al., 2006, 2008) and is an important reserve carbon source for grain yield (Bonnett and Incoll, 1992; Schnyder, 1993). WSC can accumulate in wheat stems to more than 40% of total stem dry weight (Housley, 2000). WSC mobilises from the stem during the later phase of grain filling and can potentially contribute to about 20% of grain yield under normal conditions (Wardlaw and Willenbrink, 2000). The stem WSC becomes more important for grain yield in cereal crops in drought and heat-prone environments (van Herwaarden et al., 1998; Wardlaw and Willenbrink, 2000; Barnabás et al., 2008). Variation in stem WSC concentrations among wheat genotypes is one of the genetic factors influencing grain weight and yield in drought- and heat-prone environments in Northern Australia (Xue et al., 2008b).
Fructans are soluble linear or branched β-2,1- or β-2,6-linked fructosyl-oligosaccharides and are recognised as one of the major forms of carbon reserve in about 15% of higher plant species (Hendry, 1993; Vijn and Smeekens, 1999; Van Laere and Van den Ende, 2002; Van den Ende et al., 2011). Fructans deposited in the stem and leaf sheath of temperate cereals act as short-term carbon reserve, while fructan accumulation in some dicots and perennial grasses for longer term storage (Valluru and Van den Ende, 2008). Although the primary role of fructans in plants is to bridge gaps between the excess and deficit of photosynthetic carbon relative to the carbon demand, fructans are also used for osmoregulation during flower opening (Le Roy et al., 2007), protection of plants from drought and cold stresses through membrane stabilisation (Valluru and Van den Ende, 2008; Kawakami et al., 2008; Livingston et al., 2009; Van den Ende and Valluru, 2009) and serve as antioxidant (Van den Ende and Valluru 2009; Bolouri-Moghaddam et al., 2010).
Fructans are synthesised from sucrose in vacuoles of cells by a group of fructosyltransferses belonging to plant glycoside hydrolase family 32 enzymes (Ritsema and Smeekens, 2003; Chalmers et al., 2005; Altenbach et al., 2009; Van den Ende et al., 2009). Fructans in cereals are mainly of the graminan type, that is predominantly β-2,6-linked fructosyl-units with short β-2,1-linked branches (Ritsema and Smeekens, 2003; Chalmers et al., 2005). The β-2,6-linked fructan is synthesized by the consecutive action of sucrose:sucrose 1-fructosyltransferase (1-SST) and sucrose:fructan 6-fructosyltransferase (6-SFT). The enzyme responsible for the synthesis of β-2,1-linked branches in graminan is fructan:fructan 1-fructosyltransferase (1-FFT) (Kawakami and Yoshida, 2005).
1-SST, 6-SET and 1-FFT cDNAs from wheat have been characterised (Kawakami and Yoshida, 2002, 2005). The enzyme 6G-FFT, which couples fructosyl residues to either the terminal glucose via a β-2,6-linkage or a terminal fructose via a β-2,1-linkage (Ritsema et al., 2003, 2005), is known to be absent in barley (Lasseur et al., 2011). The structure of cereal fructans suggests that 1-SST and 6-SFT are the more important enzymes involved in fructan synthesis in cereals. This appears to be reflected by the relative mRNA abundance of 1-SST, 6-SFT and 1-FFT genes in wheat stems. The Affymetrix array hybridisation signals of 1-SST and 6-SFT transcripts in wheat stems at anthesis are about 5-50 times higher than that of 1-FFT (Xue et al., 2008b).
Fructan synthesis in plants is known to be regulated by metabolic signals such as the concentration of sugars, particularly sucrose, and developmental stimuli (Blacklow et al., 1984; Müller et al., 2000; Maleux and Van den Ende, 2007; Ruuska et al., 2008; Kusch et al., 2009), as well as environmental factors such as drought and low temperature (De Roover et al., 2000; Wei and Chatterton 2001; Hisano et al., 2008; del Viso et al., 2009; Rao et al., 2011). Several studies have indicated that expression of fructosyltransferase genes in cereals is regulated by sucrose and stem developmental signals (Müller et al., 2000; Martínez-Noël et al., 2001; Koroleva et al., 2001; Nagaraj et al., 2001, 2004; Lu et al., 2002; Martínez-Noël et al., 2006; Ruuska et al., 2008). It appears that up-regulation of fructosyltransferase genes by sucrose coupled with their up-regulation at the early reproductive stage and down-regulation at the mid grain filling stage provides fine tuning in the regulation of fructan accumulation in temperate cereals. As a result, excess sucrose can be deposited as a temporary carbon reserve at the early reproductive stage when the photosynthesis capacity of the plant exceeds its carbon demand. The stored fructan mobilises through the action of fructan exohydrolases (Van den Ende et al., 2003, 2005; Kawakami et al., 2005; Van Riet et al., 2006) when the sink (grain) demand for carbon is higher than its photosynthesis capacity at the mid and late grain filling stage.
To understand how fructosyltransferases are regulated, recent studies have identified some molecular components in sucrose-mediated induction of fructan synthesis in plants such as phosphatase type 2A, protein kinases, small GTPases and phosphatidylinositol 3-kinase (Martínez-Noël et al., 2001, 2006, 2007, 2009, 2010; Kusch et al., 2009; Ritsema et al., 2009). These signalling molecules are required for the up-regulation of fructosyltransferase genes by sucrose (Ritsema et al., 2009). To further dissect the molecular basis of the high WSC trait, a genome-wide expression analysis was performed using Affymetrix wheat genome array in the stems of recombinant inbred Seri×Babax (SB) lines of wheat (T. aestivum L.) varying in stem WSC concentrations. These studies showed that the mRNA and enzyme levels of 1-SST and 6-SFT in wheat stems at anthesis are positively correlated with WSC and fructan concentrations among SB lines (Xue et al., 2008b).
Trans-acting factors involved in sucrose or developmental regulation of fructan synthesis are still unknown. Elucidation of trans-acting factors and gene regulatory networks controlling fructan accumulation is important to develop methods of identifying and producing plants with improved levels of fructan.